The invention is a novel liquid mixed triaryl phosphate composition containing a reduced concentration of triphenyl phosphate.
Mixed synthetic triaryl phosphate esters have been prepared by alkylating phenol with alkenes, such as propylene or isobutylene, to obtain a mixture of phenol and substituted phenols. According to U.S. Pat. No. 4,093,680, this alkylate mixture can then reacted with phosphorus oxychloride (POCl.sub.3) to form a mixed triaryl phosphate ester. The product mix is a statistical mixture based on the composition of the starting alkylate and always includes some fraction of triphenyl phosphate (TPP), usually from 5 to 50 percent. The product's physical properties are determined by the degree of alkylation of the phenol. A highly alkylated phenol mixture will result in a more viscous phosphate ester product than one less substituted. For example, this process results in products that are a mixture of phosphate ester isomers including: triphenyl phosphate, diphenyl alkylphenyl phosphate, phenyl di(alkylphenyl)phosphate, and tri(alkylphenyl)phosphate ("mixed alkylated phosphate esters"). It is also possible that phenol rings with 2-5 alkyl groups may be present, however, these groups are slow to react with POCl.sub.3, and are present at less than 0.5% in commercial products. The final product is tailored by selecting a specific blend of phenol and alkyl phenols as starting materials.
Such phosphate esters are used as fire resistant hydraulic fluids and lubricant basestocks, lubricant anti-wear additives, and flame retardant plasticizers. A phosphate ester that is highly resistant to oxidative, thermal, and hydrolytic degradation is desirable. In addition, a phosphate ester that has low volatility will not exhibit large evaporation losses, which is advantageous for high temperature applications.
Prior phosphate esters with good oxidative stability had poor hydrolytic stability, and conversely, phosphate esters with good hydrolytic stability had reduced oxidative stability. It is desirable to produce a triaryl phosphate ester with a combination of excellent thermal, oxidative, and hydrolytic stability.
Alkylated (iso-propyl, tert-butyl, iso-octyl, isononyl) triaryl phosphate esters are useful as fire resistant hydraulic fluids, lubricants and lubricity additives. Currently used product are prepared by reaction of phosphorus oxychloride with mixture of alkylated phenols which contain significant amount of phenol. As a consequence, the resulted product contains triphenyl phosphate at concentrations well above 5%. Triphenyl phosphate is known as mild cholinesterase inhibitor and as such should be avoided in lubricants. The prior art compositions of alkylated triphenyl phosphates with low concentration of triphenyl phosphate are of high viscosity and are not useful as hydraulic fluids.
A recent example of a disclosure that is also relevant to the present invention is contained in U.S. Pat. No. 5,206,404. The products that are described in this patent that have a lower content of triphenyl phosphate were obtained by removal of triphenyl phosphate from the standard product by distillation. The resulted product is usually of high viscosity (over 85 cSt at 40.degree. C.), and the composition contains a high concentration of multi-substituted molecules and a relatively low concentration of mono-alkylphenyl diphenyl phosphate and, as such, is not very useful in hydraulic fluid applications.
An older example of a disclosure that is also relevant to the present invention is contained in U.S. Pat. No. 2,275,041. The products that are described in this patent have a low triphenyl phosphate content and a high mono-alkylphenyl diphenyl phosphate content. However, the products described in U.S. Pat. No. 2,275,041 are too reactive to be suitable for use as hydraulic fluids where thermal stability and non-reactivity are primary performance criteria. In fact, the patentees claim their products "are useful as modifying agents for various synthetic resins, and, because of the reactivity of the allyl-type group, are particularly useful as intermediates in the preparation of a wide variety of new organic chemicals."